A metal gasket comprising a bead sheet and a coating material provided thereon by coating the bead sheet with a rubber-mixed paint comprising 100 parts by weight of fluororubber, 50-200 parts by weight of an inorganic lubricant such as graphite, carbon, and molybdenum dioxide, and 4-16 parts by weight of a silane coupling agent, followed by vulcanization, has been so far proposed as a gasket metal sheet coated with a coating material capable of maintaining high slippability and non-stickiness for a long time without any breaking or peeling problem by controlling the rubber flow (see Patent Document 1).
The object of addition of the silane coupling agent contained in the proposed rubber-mixed paint is to control the rubber flow, that is, to increase the rubber stiffness, whereas the anti-freezing liquid resistance, or the long-term durabilities such as oil resistance, heat resistance, etc. are deteriorated. The inorganic filler showing low friction factor is hard to disperse, and the particle size of the filler is consequently larger, and particularly in cylinder head gaskets, etc. in the product rubber layer thickness as small as about 25 μm the seal surface is much roughened, giving an adverse effect on the sealability. Furthermore, when the proportion of admixed amine-containing silane coupling agent exceeds 4 parts by weight on the basis of 100 parts by weight of fluororubber, the pot life of the coating solution, i.e. the fluororubber-mixed paint, is considerably deteriorated, where the productivity is substantially lost.
In the gasket raw materials generally using rubber-metal laminate sheets, metallic sheets are used upon applying thereto a chromating treatment or a chromating substitute treatment (hexavalent Cr-free treatment, or non-chromating treatment such as iron phosphatizing treatment, zinc phosphatizing treatment, etc.). However, the chromating treatment involves an environmental load problem, and rubber-metal laminate sheets produced by application of a chromating substitute treatment for the metal sheet, followed by multi-layer coating also have a heavy environmental load from the total product side, in view of the complicated treating process, and further are costwise inadvantageous.